1. Technical Field
The invention relates to voltage regulated power supplies for semiconductor based electronics and more particularly to providing a full bridge power supply with high operating efficiency and stability.
2. Description of the Related Art
Contemporary semiconductor electronic components generally operate with direct current supplied at low and closely controlled voltages. Close regulation of unipolar voltage is required because nearly all semiconductor components operate best at a constant power supply voltage. In some applications, such fine regulation is also required to minimize power loss, for example, where large variations in output current drawn by a load occur.
Switch mode power supplies are widely included among the most efficient of regulated power supplies because they provide such fine voltage regulation with little wasted power over a wide range of unregulated unipolar input voltages and output currents. Switch mode power supplies are usually connected to an unregulated, relatively high voltage direct current input source, and have one or more switching elements, an output transformer, output rectifying and filtering components, and a feedback control circuit controlling the switching element(s). The unregulated voltage level of the direct current input source is periodically applied to the primary winding of the output transformer by the switching element(s). The output off the secondary winding of the output transformer is rectified and filtered to supply power to semiconductor based electronics.
Control of the quantity of power transferred through the output transformer is done by varying the on time of the switching elements used to apply the voltage generated by the direct current input source to the output transformer. The control signals used to control the duty cycle of the switching elements are generated by the feedback control circuit in response to a direct or indirect measure of the power drawn from the output transformer. In response to that measure, the feedback control circuit pulse width modulates the gating control signals applied to the switching elements. As power drawn by a load connected to the output transformer increases, the proportion of the time the duty cycle takes increases. A decreasing duty cycle occurs with falling power demand. If the duty cycle changes without a change in power demanded, output voltages will change with potentially adverse consequences for the load electronics.
A favored type of switched mode power supply and voltage regulator arranges the switching elements in a full bridge topology, with each switching element driven by an isolated gate signal. In a full bridge, four switching elements are connected between the terminal ends of the primary winding of the output transformer and either ground or to the unregulated voltage direct current source. During each cycle, a terminal end will alternatingly be connected to the voltage source and to ground. During a portion of the period one terminal end is connected to ground the other terminal end is connected to the voltage source, and vice versa. Thus primary winding current reverses direction each half cycle. Alternating pairs of diagonally opposed switching elements in the bridge carry the current for a selected duration of each half cycle. During this period power is transferred to the load side of circuit. It is important to note that diagonally opposed pairs of switching elements are not turned on and off together, but in a phase staggered fashion.
Metal oxide semiconductor field effect transistors (MOSFETs), designed for power applications, are commonly used as the switching elements. See for an example of such an application, U.S. Pat. No. 4,758,941. MOSFETs are favored as switching elements in power applications because they work well as saturated devices, exhibit high input impedances and have a good thermal stability compared to bipolar junction transistors. The first two factors contribute to switching efficiency, which directly contributes to power supply efficiency.
One disadvantage of MOSFETs is that they have high drain to source capacitances. As a result, MOSFETs exhibit capacitive switching losses, which accumulate with increased switching frequency. Unfortunately, increasing switching frequency is desirable because it improves power pulse density for fine regulation of output voltage. As a result, operating efficiency of the power supply is directly and adversely affected by increasing switching frequencies. Capacitive switching losses can be reduced by switching MOSFETs at a zero voltage drop across the MOSFETs. However, simple application of zero voltage switching (ZVS) is complicated by other considerations.
As observed above, detection of power drawn from an output transformer may be direct or indirect. In the full bridge topology, determining the current in the primary winding of the output transformer is desirable for current mode control of the feedback control circuitry. The current in the primary winding reflects current in the secondary winding and thus is related to power demanded by the load connected to the rectifying and filtering circuitry. However, because current freewheels through the primary winding of the output transformer during switching, placing a current sensing transformer in series with the primary winding, or use of the primary winding itself for current sensing, does not work. The problem is that the current sensing transformer never sees zero current, preventing flux in the transformer from being reset.
A full bridge power supply and voltage regulator has four arms, two of which carry the current through the primary at any given time. No current is carried by a given arm in the bridge at least half the time. Current sensing can be done in the arms which allows for reset of the sensing transformer during the zero current periods. In application, two current transformers are used for current sensing. The transformers are located in one of the two legs of the bridge. A bridge leg includes two arms connected between the unregulated voltage direct current source and ground and directly connected to one another. When the switching element in one arm of the leg comes on in a half cycle of the bridge, the switching element in the other arm of the leg is off. If each arm of a leg has a current sensing transformer, one of the current transformers is active with each half cycle. With current sensing done in the arms of one leg, flux in the current sensing transformer in the quiescent arm can be reset.
The physical topology of a full bridge voltage regulator exhibits perfect mirror symmetry. Thus it may be surprising to the reader that the currents conducted by the legs do not exhibit the same symmetry. The phase relationships of the control or gating signals supplied by the feedback control circuitry to the MOSFETs result in the legs conducting different amounts of current. Phase staggered switching results in one leg of the bridge being a leading leg and the remaining leg being a lagging leg. An arm in the leading leg conducts positive current (by "positive is meant current in a direction opposite to the conducting orientation of the body diode of the switching power MOSFET) over its entire on period, following a brief negative current pulse in the body diode of the MOSFET. Net current in an arm of the leading leg over a cycle is positive. An arm in the lagging leg conducts negative current through its body diode during most of the freewheeling interval (corresponding to "off-time" of a conventional pulse width modulated regulator) and positive current as a new power pulse begins (i.e. the "on-time" of a conventional pulse width modulated regulator). Net current in an arm of the lagging leg can be positive or negative.
To use current mode sensing, a decision must be made as to which leg to place the current sensing transformers. Preferably, the decision should be based on the current profiles of the arms in the leading and lagging legs, respectively. On this basis, placing the current transformers in series with the MOSFETs in the arms of the leading leg would be desirable. One reason for this is that leading leg current is on net positive. This would allow use of passive reset networks for the current sensing transformers. For a current sensing transformer placed in an arm of the lagging leg, net current through the current sensing transformer can be positive or negative, which in turn requires an active network to determine the resulting orientation of the residual flux and to reset the flux to zero. An active reset circuit is more complex, more costly and uses more power than a passive circuit.
Another reason to locate current sensing transformers in the leading leg is that the resonant transition that occurs when a MOSFET switches is much slower for a MOSFET in the leading leg than for one in the lagging leg. This is because the only energy available to charge the drain to base capacitance of the MOSFET in the leading leg comes from the leakage inductance of the power transformer. For MOSFETs in the lagging leg, energy is also available from the output filter connected to the output transformer, which causes faster resonant transitions. The adverse consequences of adding parasitic inductance to a leg by insertion of current transformers is reduced if slower current transitions are seen.
Finally, were current sensed in the arms of the leading leg, it would be more nearly continuous. It is often desirable to generate a signal proportional to the output current of the power supply for diagnostic uses. Were leading leg current sensing used, a proportional current signal could be recovered with a simple resistor/capacitor filter (RC filter) connected to the current sense resistor. Where current sensing is done in the lagging leg, an active circuit has been used for peak detection because the feedback control signal is pulse width modulated.
Despite the advantages of leading leg sensing, circuit designers have placed current sensing transformers in the lagging leg of full bridge regulators. This is done because the signal recovered from the leading leg is not well suited for current mode control.
Leading leg current sensing has resulted in circuit instability. This undesirable result overrides all other factors in design consideration. Power supply circuits are feedback controlled circuits which use the current sense signal from the current transformers as the basis for a feedback control wave. Current in an arm of the leading leg is continuous from the on period into the following freewheeling interval, exhibiting only a change in slope. A control signal generated directly from such a current reflects the slopes of the current. Contemporary pulse width modulation controllers act as a source of a ramp current into current loop on the output side of the current transformers. This is required to stabilize the circuit. However, the ramp current changes the slope of the sensed current signal and can make detection of the peak of the control signal difficult if not impossible. Detection of the peak is essential because knowledge of its timing tells the pulse width controller how much power is being transferred through the output transformer. The positive slope of the ramp signal can cancel the negative slope of the control signal corresponding to the freewheeling phase of operation in the leading leg, obfuscating the peak. Under this condition a power supply can easily become unstable. Power duty cycle time can wander resulting in voltage transients across the load and possible damage to the load circuitry. By using lagging leg current sensing, a clean peak is obtained and stability problems are avoided.